Vapor generator



Sept. 26, 1 967 w. P. GORZEGNO ETAL 3,343,523

VAPOR GENERATOR 5 Sheets-Sheet 1 Filed Oct. 22, 1965 WALTER P. GORZEGNO JACOB COOPER BY MM! M44 Sept; 26, 1967 w. P. GORZEGNO ETAL 3,343,523

VAPOR GENERATOR 5 Sheets-Sheet 2 Filed Oct. 22, 1965 Fig.2

lA/Vf/VTOES WALTER R GORZEG'NO --:@iLF---- l I l l ll nnnu l JACOB COOPER A 7 OR/V6 Y Sept. 26, 1967 w. P. GORZEGNO ETAL 3,343,523

VAPOR GENERATOR 5 Sheets-Sheet 5 Filed Oct. 22, 1965 Pass no.4

INVENTORS WALTER P. GORZEGNO JACOB COOPER Poss no. 3

ATTORNEY Sept. 26, 1967 w. P. GORZEGNO ETAL 3,343,523

VAPOR GENERATOR 5 Sheets-Sheet 4 Filed Oct. 22, 1965 230m 3.. 5 5m 255 o m o w l/VVVVTO/QS WALTER P. GORZEGNO JACOB CO0 PE R W l M A TTOAIVE Y P 1967 w. P. GORZEGNO ET'AL 3,343,523

VAPOR GENERATOR 5 Sheets-Sheet 5 Filed Oct. 22, 1965 INVEN TOES WALTER P. GORZEGNO JACOB COOPER A TTOENEY United States Patent 3,343,523 VAPQR GENERATOR Walter P. Gorzegno, Florham Park, and Jacob Cooper,

Livingston, N.J., assignors to Foster Wheeler Corporation, New York, N.Y., a corporation of New York Filed Oct. 22, 1965, Ser. No. 501,269 17 Claims. (Cl. 122-406) This invention relates to a forced-flow supercritical and subcritical once-through vapor generating unit.

More particularly, the invention relates to a furnace circuit arrangement for forced-flow units in which the furnace tubes are vertically aligned in parallel and adjacent tubes are fin-welded along their length to form a gas-tight, tubular panel wall, furnace enclosure.

It is known to construct a forced-flow unit with a large number of parallel tubes welded together to define a gastight enclosure. The tubes are connected between inlet and outlet headers. One problem involved with this construction resides in achieving equal flow distribution among the tubes. For instance, such factors as an unbalance in firing of burners, slag accumulation, or differences in tube lengths may cause a flow unbalance resulting in higher heat absorption per lb. of fluid in some tubes than others. This condition may result in such disadvantages as excessive metal temperatures, or a cycling of metal temperatures in these boundary wall tubes heated from one side, making fatigue failure probable.

In accordance with the invention, it has been discovered that these disadvantages can be overcome by dividing the generator furnace wall enclosure, in the lower high temperature radiant zone, peripherally into at least three side-by-side pass sections, each pass section comprising parallel finned tubes welded along their lengths into vertically oriented panels. Inlet and outlet headers serve each pass section separately, and are connected so that the flow in the enclosure Wall is in series through the successive pass sections. Since the temperature of the fluid increases from the first to the last pass section, the pass sections are arranged so that the intermediate section is divided into two panels operating in parallel and disposed between the panels of the first and last sections.

The panels for all the pass sections are welded together as with the tubes, so that the enclosure is gas tight.

By the invention, the flow in tubes of a pass section is made sufficiently uniform to avoid tube overheating and limit fatigue causing temperature fluctuations in any one pass. A temperature differential will exist between adjacent welded tubes of adjacent pass sections, but, in accordance with the invention, the arrangement of the intermediate pass section panels between the first and last section panels achieves a temperature differential between tubes of the different pass sections less than 100 F. and acceptable with respect to weld and tube strength design criteria.

The invention and advantages thereof will become more apparent on consideration of the following description, and accompanying drawings, in which:

FIGURE 1 is an oblique expanded View of a subcn'tical forced-flow once-through vapor generator in accordance with the invention;

FIGURE 2 is a side elevation view of the generator of FIG. 1;

FIGURE 3 is a further exploded view showing in detail elements of the generator of FIG. 1;

FIGURE 4 is a section of the panel wall of the generator of FIG. 1;

FIGURE 5 is a temperature vs. enthalpy diagram for the subcritical generator of FIGS. 14 for maximum continuous load at 3.547 million pounds of steam per hour; and

FIGURE 6 is an oblique expanded view of a forcedflow once-through vapor generating supercritical unit in accordance with an embodiment of the invention.

Referring to FIGS. 1-3, the vapor generator includes a furnace 12 for a forced circulation steam generating unit which has a rectangular horizontal cross-section and is vertically orientated. Burners 14 and 16 (schematically indicated in FIG. 1) are disposed in the lower portion of the furnace enclosure in the front furnace wall 18 and rear furnace wall 20 respectively. Above the lower portion of the furnace enclosure, the front and side walls continue upwardly to define an upper portion 22 of the furnace enclosure, from which a gas pass above the rear wall 20 leads to the convection portion 23 of the generator. In this respect, the flue gases from the combustion of a suitable fuel leave the furnace enclosure by passing over a furnace exit screen 24 flowing, in cross-flow relationship, first over a platen superheater 26 in the high heat absorption area of the upper furnace (in front of screen 24) and then over the finishing superheater bank 27 between the front screen 24 and a rear screen 28. The convection section of the unit includes an economizer pass 30, a reheater pass 32, and a primary superheater pass 34. The flue gases exiting the convection section flow through an air heater and to the stack (not shown).

The high pressure fluid flow circuitry routing through the furnace enclosure of the unit (upper and lower portions) consists of five series connected pass sections joined through mixing headers. In general, floor pass No. l, and upflow passes Nos. 2, 3 and 4 form the floor and enclosure walls in the lower portion of the furnace, and upflow pass No. 5 forms the enclosure walls in the upper portion of the furnace. All furnace tubes are Welded along their lengths as shown in FIG. 4 forming fin-welded panels and a gas-tight construction.

Specifically, feed water enters inlet 30a to the economizer 30 (which is disposed in the flue gas passages therefor) and flows from the economizer via two downcomer pipes 30b to feed the furnace floor pass No. 1, item 36. The fluid from floor pass No. l is transmitted then to an inlet header 38a feeding the tubes of pass No. 2 (item 38) comprising a substantial portion of the front wall 18. From the outlet header 38b for the second pass (about halfway up the furnace enclosure), the fluid flows in parallel downcomers 39 to the separated L-shaped headers 40a feeding opposed third pass panels, items 40. These L-shaped panels make up the remainder of the front Wall 18 on opposite sides of the second pass panel 38, and more than half of each side wall.

The fluid from furnace pass No 3 exits via opposed upper L-shaped headers 40b to downcomers 41 feeding a lower U-shaped header 42a which constitutes the inlet for U-shaped panel of pass No. 4 (item 42). This panel makes up the rear portions or remainder of the side walls and the rear wall. A mixing bottle 43 assures a uniform enthalpy flow to the fourth pass lower header. From upper header 42b for this pass, fluid is transmitted by risers 44 to a U- shaped header 46a feeding the fifth pass 46. This pass makes up the side and front walls of the upper portion of the furnace enclosure, with U-shaped outlet header 46b also serving the pass.

From the outlet header 46b, a plurality of riser tubes 47 transmit the flow to opposite downcomers 48 leading to Ts 50 (on opposite sides of the boiler), from which lines 52 and 54 transmit the flow in desired proportions to (a) the enclosure for pendant finishing superheater 27, and (b) the enclosures for the convection section, economizer, reheater and superheater passes 30-34, respectively. For the pendant superheater enclosure, the flow in line 52 divides into lines 56 and 58 feeding the front and rear screen tubes 24- and 28 and the enclosure side walls 60, respectively. For the convection passes, the flow feeds the rectangular shaped header 62 near the convection pass outlet. This header is provided with a cross-pipe 64 which feeds the division wall 66 for the convection pass. From all of these passes, the flow leadsto a common header 68 at the top of the unit.

From the header 68, risers 70 feed inlet header 72 for the roof tubes 74, the fluid flowing in sequence to outlet header 76 for the roof tubes, to the primary superheater 34 via inlet header 78 therefor, and to the platen superheater 26 via outlet header 80 for the primary superheater and inlet header 82 for the platen superheater. Item 84 is the outlet header for the platen superheater and feeds the finishing superheater bank 27 exiting at header 86.

The reheater 32 is fed by inlet header 90, with flow countercurrent to the gas flow to outlet headers 92 and 94.

Many advantages including the following are gained.

Division of the furnace periphery into a plurality of passes in series limits the extent of flow unbalance which can occur in these forced flow circuits by virtue of heat absorption variation. Where the furnace enclosure is made up of a single circuit between mix headers, such factors as unbalances in firing, difierences in tube lengths, and slag deposition can cause severe flow variations in this multi-tube circuit. Severe flow unbalance in such a single circuit would then cause inadequate cooling and overheating of some tubes.

' In accordance with the invention, confining a pass to several hundred tubes, and limiting the fluid enthalpy pick-up in the pass, particularly in the high absorption area of the furnace (in the area of the burners), overcomes the tendency of this maldistribution of the flow, thereby avoiding excessive tube wall metal temperatures.

A particularly critical area in the furnace circuitry (in a subcritical unit) is the mixture (steam and water) cooled region of the circuitry. In a high heat absorption zone, when the steam content in the flow becomes high enough, a continuous steam film may cover the inside of the tube wall causing departure from desired nucleate boiling and resulting in a low heat transfer rate to the fluid. This in turn results in tube overheating. Maintaining nucleate boiling depends in part on maintaining a turbulentflow in the tubes. In the mixture cooled region (which in the embodiment shown may be in pass No. 4), the multiple pass design of the invention permits sizing this region or pass so that a predetermined minimum flow rate is maintained in the tubes of the pass. A turbulator device (disposed in tube lengths as shown in FIG. 4) also is used. This device, with flow rates above a specified minimum, causes a proper turbulent flow condition in the tubes to prevent a departure from nucleate boiling, even at high absorption rates and at high proportions of steam by weight in the tubes. By proper design of this mixture pass with installed turbulators, nucleate boiling is maintained over the full operating range of the unit.

' As an additional advantage, minimum temperature variations in an upset tube are experienced thereby limiting cycling of stresses and improving fatigue life. With a single furnace pass or circuit, the enthalpy pick-up in the pass or circuit is many times that per pass Where many passes make up the furnace enclosure. Studies have shown that differences in tube metal temperatures between an upset tube and average tube increase markedly with increase of circuit enthalpy pick-up, so that with a single furnace circuit having a high enthalpy pick-up, excessive cycling of stresses in the all-welded enclosure are likely, resulting in curtailed fatigue life. The present invention is an improvement in dividing the enclosure into a plurality of in-series passes each with a lesser enthalpy pickup.

Further, to limit enthalpy pick-up per pass and to avoid flow unbalance, the arrangement of passes permits dividing the furnace vertically, in its longitudinal direction, into upper and lower portions with the high absorption area passes Nos. 2, 3, and 4 terminating about half way up the furnace and being of approximate equal length. In this respect, if the furnace passes extended the full height of the furnace, the enthalpy pick-up per pass would be high, and those tubes in the rear wall would be of different length than the tubes of the remainder of the enclosure (to provide a passageway for the furnace gases to the convection section of the unit), both factors contributing to the tendency to maldistribution of flow.

As a general rule, the mix headers between lower pass No. 4 and upper pass No. 5 are about halfway up the furnace, but are located sufiiciently high up to have a heat input into the upper pass reduced to the extent that the tubes of the upper passes with lower fluid mass flow rates can be still properly cooled.

In the unit in question, the firing is by opposite rows of burners in the front and rear walls. Because of the arrangement of the lower furnace passes, and the limited fluid enthalpy pick-up of all furnace passes, this design can tolerate significant firing unbalance without detrimental effect. For example, a front to back firing unbalance will increase overall absorption in the front or rear wall pass but will not significantly upset absorption in any given pass to cause damaging circuit flow unbalance. However, in this subcritical design, the front to back firing unbalance should not exceed that which would cause a steam water mixture entering pass No. 4 to exceed 8% steam by weight. For balanced firing this fluid is a subcooled single phase entering pass No. 4. A side to side firing unbalance will cause an upset absorption for all furnace passes, in particular the side walls (pass N0. 3), but because of the limited pass fluid enthalpy pick-up, a significant absorption upset will only cause a minor unbalancing of circuit flow.

As a further advantage, the use of a plurality of passes in series in the furnace enclosure as described allows and in fact requires the use of larger tubes than conventional in once-through boilers. As the tubes of a pass are reduced in number from a few thousand to several hundred, the mass flow rate (lbs./hour/sq. ft.) per tube is correspondingly increased. The need for larger tube diameters is apparent. In the unit in question, the tubes are all 1% inch CD. (which is large for a once-through boiler) with .180 to .200 inch minimum wall thickness, on 1 /2 inch centers. The use of larger tubes, which are relatively stiffer, makes it easier to weld the tubes into panels, makes it easier to design and install turbulators, and has other advantages, which in turn achieve economies in construction of the unit.

In practice, it is contemplated that the specific design shown will be used with subcritical pressures. In such a unit, there are definite phases of water, water and steam,

and steam with the problem of maintaining nucleate boiling, as mentioned, in the steam-water mixture pass located in the lower high absorption area of the furnace. In this respect, it is desirable that a water phase exist at least at the inlet of every lower furnace tube. To insure the existance of a water phase at the pass No. 4 tube inlets, the pass is designed with as large a number of tubes as possible (consistent with maintaining minimum mass flow in the pass). In other words, the tubes of passes Nos. 2 and 3 make up as little of the furnace periphery as possible. Thus, as a design criteria, the enthalpy pick-up in the circuit-up to pass No. 4 is less than that which will with balanced firing create a steam and water phase at the inlet to this pass.

As a criteria for proper operation of the turbulator'device, a mass flow of X 10 lbs./hr./sq. ft is considered minimum in a tube in pass No. 4 in the high heat absorption zone of the furnace. The disposition of tubes in enclosure passes Nos. 2-4, assures that a water phase enters this pass. At the same time, the number of tubes in pass No. 4 is sufliciently restricted to assure that at least the minimum mass flow is obtained in the tubes of this pass.

The number of tubes in the floor pass No. 1 is of course dictated by the width of the unit.

6 No. 6 the fluid then flows through the roof tubes 178, then to the platen and finishing superheaters.

In some design arrangements, it is possible to combine series connected passes Nos. 5 and 6 into one upflow fluid pass.

As with the subcritical unit, the steam generator illustrated is top supported by suitable steel members (not shown) permitting free expansion. The furnace upper tubular elements (of passes Nos. 4 and 5) are suspended from the top, and the lower passes are suspended from the upper passes by interconnecting adjacent tubular elements of the respective passes in the manner set forth in TABLE I Heat Pick-up Btu/lb. Fluid Outlet Temp, F. Location No. of 100% 100% 30% Tubes Load Load Load Load Pass No. l Floor 429 585 525 Front Wall 382 80 640 605 Front and Side Walls- 390 80 680 665 Side and Rear Walls 664 135 180 695 67 5 Pass No. 5 Upper Furnace Front and Side Walls- 1, 006 80 95 690 675 FIG. 6 illustrates principles of the invention specifically for a supercritical steam generator, wherein the numbers 112 and 114 represent the furnace and convection enclosures respectively, the latter containing economizer 116. From the economizer, downcomers 118 lead to the ends of a I-shaped header 120, the stem of which feeds a wall of tubes 122 which makes up the front slopping hopper pass 124 of the furnace and the lower part of the front Wall 126. The legs of the header feed opposed tubes 124 which make up a portion of each side wall of the furnace. The tubes comprising these side wall portions and the front Wall are integrally joined to form a U-shaped panel constituting pass No. 1 of the furnace circuits. At the top of the pass No. 1 the tubes feed to a U-shaped header 120a, from which the fluid flows to two externally disposed downcomers each of which feeds to the inlet of furnace pass No. 2 via opposed inlet headers 132. This pass is composed of opposite side wall panels 134, abutting and welded to the free ends of the U-shaped pass No. 1 panel. At the top of these panels, the tubes feed into two outlet headers 136 disposed in opposite sides of the furnace.

From outlet headers 136, the fluid flows downward through two external downcomers 140 into a U-shaped entrance header 142 for pass No. 3. The tubes of pass No. 3 extend therefrom and form a U-shaped panel 144 comprising the lower part of the rear wall 146 and portions 148 of the side walls adjacent thereto. The fluid flows upward through the tubes of pass No. 3 and feeds into U-shaped exit header 150. This panel is welded to b tubes of the pass No. 2 panel so that the entire enclosure is gas tight.

From the header 150, the fluid routes to a U-shaped inlet header 152 of furnace pass No. 4 via piping 154. The tubes of pass No. 4 extend upward from header 152 and make up the entire side walls and front Wall of the upper furnace region.

From outlet header 156 for pass No. 4, at the top of the furnace, the fluid exits through downcomers 158 to feed inlet header 160 for a pass No. 5, which forms the upper part of the rear wall 162 of the upper furnace enclosure, and the pendant vestibule enclosure for the superheater bank 168. It comprises furnace exit screen 170, rear screen 172 and the side and bottom walls 174 and 176, respectively, of the enclosure.

From pass No. 5, the fluid is routed through a suitable downcomer to feed pass No. 6 which includes the front, rear, side, and partition walls of the enclosure 114 for the horizontally oriented convection surface. From pass copending application, Ser. No. 370,604, filed on May 3, 1962. Since pass No. 5 is aligned above, and supports pass No. 3, a minimum temperature differential between tubes of the upper and lower furnace passes is achieved, than would be the case if the lower (or upper) furnace passes were reversed and pass Nos. 1 and 5 were adjacent.

As an alternative, it is possible to provide an additional intermediate furnace pass in the lower furnace portion formed of finned-welded tube panels disposed on opposite side walls adjacent the above described pass No. 2 panels. In other respects, the unit would be similar to the unit heretofore described and shown, except that it has one extra pass. In addition, pass No. 5, as described above, may be extended to encompass a portion of the furnace periphery occupied by pass No. 4, thereby reducing the periphery of the furnace cooled by pass No. 4, and permitting the mass flow rate in this pass to be increased.

For the forced flow once-through generating units described, suitably sized pipe connections are used to connect the multiple passes, the pass outlet and inlet headers being properly sized to limit fluid flow unbalance caused by velocity head and pressure drop variations in these headers.

It should be noted that, as with the subcritical unit, the passes are sized to keep maximum fluid flow velocity in the tubes of those passes where it is most needed. The fluid passes in the furnace walls are arranged to assure that flow velocities within the tubes are kept highest at locations where maximum heat absorption is obtained. In addition, in a high absorption zone such as the lower furnace, in a supercritical design, it is advantageous to design higher fluid mass flow rates in those passes where the fluid enthalpy is greatest. Therefore, available circuitry pressure drop is utilized where it is most needed. Metal temperatures are kept low (note FIG. 5) thereby circumventing the use of expensive alloy steels.

For example, from FIG. 6, pass No. 1 has more tubes than pass No. 2 which in turn has more tubes than pass No. 3. In this respect, mass flow is greatest in progressively succeeding lower furnace tubes to lower the steam film temperature drop to compensate for higher bulk fluid temperature in the succeeding passes. In the upper furnace region, the heat absorption is considerably less; accordingly, pass No. 4 is sized with a larger number of tubes and has a lower fluid mass flow rate.

Accordingly, the passes are designed for optimum fluid cooling, mass flow1bs./hr.square feet-to effect eflicient cooling of the tube metal to low design metal temperatures whereby minimum fluid pressure drop may be" achieved. Representative mass flows are asfollows:

TABLE-4 PASS FURNACE G (mass flow) The supercritical unit has been described with reference to a V-shaped hopper bottom. The foregoing principles are equally applicable to flat bottom furnaces which would be used if the unit were completely gas fired. In this case, the flat floor becomes a separate furnace pass. Similarly, the invention is applicable to a unit having a steam or water cooled furnace division wall which may be used when the unit is coal fired. A subcritical unit would employ a steam cooled division wall, and a supercritical unit a water cooled division wall.

Further advantages should now be apparent. Because each furnace pass is restricted to a portion of the furnace walls, and has low enthalpy pick-up, the temperature differentials between different passes at various loads are maintained within satisfactory limits even with extreme unbalanoes in firing, allowing a maximum tolerance for uneven heat absorption around the periphery of the furnace. In no section of the furnace does the difference of temperature between adjacent tubes of dilferent passes exceed 100 F. By limiting this temperature differential to 100 F., resulting metal stresses are kept to accepted values. Also these pass joints occur only at certain locations and are few in number. Temperature differentials between tubes of the same pass are nil, and excursions of tube metal temperatures in a given furnace wall area because of upset absorption cannot be experienced to any great extent.

The lower furnace passes having different bulk fluid temperatures terminated away from the corners of the furnace enclosure. By making the welded joint in the straight section of the wall, an improved structural design is achieved. Welding diflerent pass panels at the corners would give a less satisfactory jointure from a stress and structural point of View.

Differences between the subcritical and supercr'itical design can be emphasized. Subcritical units require that a much greater amount of the furnaceperiphery be taken up by the last lower furnace pass. In such units, excessive front to back absorption unbalance is not desirable to avoid raising the enthalpy of the fluid to the point where there is a steam and water mixture entering the last lower furnace pass. However provisions are made in the connections to this steam-water pass inlet to permit up to 8% steam by weight fluid mixtures to enter without exceeding design margins. A supercritical unit does not have this front to back absorption upset limitation. Accordingly, in subcritical units, the respective passes in the lower furnace are sized to avoid this steam-water condition entering the last pass in the lower furnace. In supercritical units, on the other hand, the lower furnace enclosure passes are sized to achieve progressively greater flow rates per tube, with the tubes of pass No. l exceeding in number those of pass No. 3. HoW-. ever, in some designs the lower furnace passes may be sized for equal fluid mass flow rates. The side to side unbalances in subcritical units are provided for in the same manner as in the supercritical units. By utilizing the present invention in subcritical units, there is provided a built-in ability to withstand reasonable front to back unbalances and extreme side to side unbalances effectively maintaining temperature differentials and excursions, and resulting stresses and stress cycling within satisfactory limits.

- The above description for the subcritical and supercritical units emphasize one major advantage .of the invention, the flexibility in design oflfered. With practically the sole limitation that sufiicient passes be provided to meet velocity and enthalpy pick-up requirements, the passes can otherwise be adjusted to any number for such purposes as avoiding the use of excessively small diameter tubes, or for avoiding the use of orifices (through frequent mixing and limited enthalpy pick-up per pass), whether the unit be supercritical or subcritical, and regardless of fuel used. In this respect, the frequent mixing approach and flexibility in number of passes also avoids the need to resort to the use, as in conventional units, of gas recirculation fans and recirculation pumps to protect the furnace during start-up and low load periods.

To offer further flexibility, the Buffer Circuit of copending application Ser. No. 501,168, filed Oct. 22, 1965 by Walter P. Gorzegno may be used in either the subcritical or supercritical designs.

It will be apparent to those skilled in the art that changes may be made in the form of the apparati disclosed Without departing from the spirit and scope of the invention as covered by the fol-lowing claims.

What is claimed is:

1. A once-through vapor generator comprising a rectangular vertically oriented furnace enclosure;

the enclosure comprising side-by-side panel sections defining at least three upflow flo-w passes;

each panel section comprising parallel vertically oriented finned tubes welded together;

the panel sections being welded together so that the enclosure is essentially gas-tight; burner means radiantly heating said enclosure; header means connecting the flow passes in series, the enthalpy of the fluid increasing in successive passes;

the panel sections being arranged whereby an intermediate enthalpy pass comprising multiple panel sections is disposed on opposite sides of and intermediate the panel sections of the higher and lower enthalpy passes.

2. A once-through vapor generator comprising Wall means defining a rectangular radiant heating section;

the wall means comprising a plurality of parallel finned tubes welded together longitudinally to define an essentially gas tight enclosure;

headers for said tubes;

the tubes and headers therefor being arranged so that the enclosure is divided into at least four side-byside panel sections, the panel sections being connected in series to form at least three upflow fluid passes of increasing enthalpy;

burner means radiantly heating said wall means;

the intermediate pass of intermediate enthalpy comprising at least two of said panel sections wherein the two panel sections separate and are on opposite sides of the remaining two panel sections of the passes of higher and lower enthalpy.

3. A once-through vapor generator comprising four side-by-side vertically oriented panel sections welded together to form a rectangular furnace enclosure;

each panel section comprising parallel vertically ori-.

ented finned tubes welded together so that the enclosure is essentially gas-tight;

burner means radiantly heating said enclosure;

means connecting the panel sections in a series relationship so that a fluid makes at least three upflow passes in said enclosure, the enthalpy of the fluid increasing in successive passes;

the intermediate pass of intermediate enthalpy comprising t-wo of said panel sections wherein the two panel sections separate and are on opposite sides of the remaining two panel sections of the passes of higher and lower enthalpy.

4. A vapor generator according to claim 3 wherein the generator is to be operated at subcritical pressures,

the panels of the lower and intermediate enthalpy passes occupying a limited portion of the furnace enclosure so that a water phase enters the higher enthalpy pass.

5. A vapor generator according to claim 4 wherein the higher enthalpy pass occupies a sufiiciently limited portion of the furnace enclosure to obtain a mass fiow of at least 1.65 lbs./hr./sq. ft. in the tubes of the higher enthalpy pass.

6. A vapor generator according to claim 4 wherein the arrangement of panel sections in the enclosure is symmetrical.

7. A vapor generator according to claim 6 wherein the panel sections for the higher and lower enthalpy passes are U-shaped with tubes in the rear, front and side walls respectively.

8. A vapor generator according to claim 4 which includes turbulators in the higher enthalpy pass tubes.

9. A vapor generator according to claim 3 wherein the side-by-side panel sections occupy a lower portion only of the furnace enclosure, further including a further upfiow pass comprising front and side wall panel sections occupying the upper portion of the furnace enclosure;

header means to transmit fluid from said higher enthalpy pass to the further pass;

the lower portion panel sections terminating on substantially the same horizontal plane.

10. A vapor generator according to claim 9 wherein the lower portion panel sections extend about halfway up the furnace enclosure.

11. A vapor generator according to claim 3 wherein the panel sections are proportioned so that a water phase enters the higher enthalpy pass, the passes each having a mass flow of at least 1.65 10 lbs./hr./ sq. ft.

12. A vapor generator according to claim 11 wherein the panel sections terminate on substantially the same horizontal plane about halfway up the furnace furnace enclosure, further including a further upflow pass comprising U-shaped front and side wall panel sections occupying the upper half of the furnace enclosure;

the tubes having approximately 1% inches OD. and

.180 to .200 inch minimum Wall thickness.

13. A once-through vapor generator comprising first wall means defining a furnace radiant heating section;

second wall means defining a convection section in gas flow communication with the furnace section; the first wall means comprising at least four side-byside panel sections, each panel section comprising parallel finned tubes welded longitudinally together,

the panels being connected in series to form at least three fluid passes of increasing enthalpy; the intermediate pass of intermediate enthalpy comprising at least two of said panel sections arranged so that the remaining panel sections of the first and third passes are separated on opposite sides by the two panel sections of the intermediate pass. 14. A forced-flow once-through vapor generator comprising;

means defining a lower furnace chamber; said means including a plurality of panels formed together each having substantially vertical oriented parallel upflow tubes welded together along their length to form a gas-tight lower furnace enclosure;

the enclosure comprising a first flow pass, a last flow pass, and at least one intermediate flow pass arranged in series flow, with the first and last flow pass;

the intermediate flow pass comprising two such panels disposed in spaced relationship from each other along the periphery of the enclosure and between the immediately preceedin'g and succeeding series arranged fiow pass panels;

mixing means between the passes arranged to receive and mix all the flow from each pass; and

external unheated downcomers between the passes for conducting the fiow from each pass to the immediately succeeding pass.

15. A forced-flow vapor generator of claim 14 wherein the panels form a rectangular enclosure and are adapted so that the vertical edges of adjacent pass panels are not coincident with the corners of said enclosure.

16. A forced-flow vapor generator of claim 14 where in the generator is designed for supercritical pressures, succeeding series passes comprising progressively increasing numbers of tubes, the number of tubes in each pass being selected so as to optimize the pressure drop and mass fluid flow therethrough.

17. A forced-flow vapor generator of claim 14 further comprising two separate inlet headers and two separate outlet headers respectively arranged to pass flow into and out of the two panels of the intermediate fiow pass.

References Cited UNITED STATES PATENTS 2,989,036 6/1961 Hake et a1.

3,159,146 12/1964 Rudolph 1226 3,162,179 12/ 1964 Strohmeyer 122406 3,247,830 4/1966 Vogler 122-406X CHARLES I. MYHRE, Primary Examiner. 

1. AN ONCE-THROUGH VAPOR GENERATOR COMPRISING A RECTANGULAR VERTICALLY ORIENTED FURNACE ENCLOSURE; THE ENCLOSURE COMPRISING SIDE-BY-SIDE PANEL SECTIONS DEFINING AT LEAST THREE UPFLOW PASSES; EACH PANEL SECTION COMPRISING PARALLEL VERTICALLY ORIENTED FINNED TUBES WELDED TOGETHER THE PANEL SECTIONS BEING WELDED TOGETHER SO THAT THE ENCLOSURE IS ESSENTIALLY GAS-TIGHTG BURNER MEANS RADIANTLY HEATING SAID ENCLOSURE HEADER MEANS CONNECTING THE FLOW PASSES IN SERIES, THE ENTHALPY OF THE FLUID INCREASING IN SUCCESSIVE PASSES; THE PANEL SECTIONS BEING ARRANGED WHEREBY AN INTERMEDIATE ENTHALPY PASS COMPRISING MULTIPLE PANEL SECTIONS IS DISPOSED ON OPPOSITE SIDES OF AN INTERMEDIATE THE PANEL SECTIONS OF THE HIGHER AND LOWER ENTHALPY PASSES. 